This should avoid random unrelated crashes.

* Improved main.py tests

* Small fixes

* Fix IA_PD length error

I tested this and noticed wireshark wasn't parsing the message correctly. Turns out the length is wrong.

* Update regression.uts

Fix test for DHCP6OptIA_PD

* Update length of DHCP6OptIA_PD in test.

This PR makes it possible to check if several flags are all set, by
writting, for example, `if pkt[TCP].flags.SA:`.

It only works for single letter flags (e.g., will work for TCP().flags
but not for IP().flags), and only when getting a value.

Thanks @guedou for suggesting that.

It is now possible to write `pkt[TCP].flags.S = True`.

This makes it possible to write `if pkt[TCP].flags.S: [...]`.

A new test that triggers the bug (with ARPSourceMACField) has been added.

Fix IPv6 support on Windows

  - LLTDAttribute() produces a Raw instance (or crashes, when
    conf.debug_dissector is set).

  - str(LLTDAttribute(type=18)) crashes.